And it's really bad if you leave it on the top shelf of your pantry for years on end.REFRIGERATE REFRIGERATE REFRIGERATE
When I go to a new bar my initial judgment of a place is whether they refridgerate their vermouth or not. Unrefridgerated low quality vermouth most bars stock is why people hate vermouth. Its nasty. I agree with Lognar I have had opened bottles refrigerated for a long time that havnt really shown signs of change but unrefridgerated vermouth will start to change in a few days.
To that end, 375ml bottles and fridge.